Active energy ray-curable composition and lens sheet

ABSTRACT

An active energy ray-curable composition comprising 
     (A) 20 to 80 parts by weight of a compound represented by the following general formula I: ##STR1## wherein R 1  represents hydrogen or methyl, X and Y may be the same or different and represent methyl, chlorine, bromine or iodine, and t and u each independently represent an integer of 0-2, 
     (B) 20 to 80 parts by weight of at least one compound having at least one acryloyl or methacryloyl group in the molecule, or (B-1) 10 to 90 parts by weight of at least one compound having at least two acryloyl or methacryloyl groups in the molecule and (B-2) 1 to 90 parts by weight of at least one monoacrylate or monomethacrylate compound having one acryloyl or methacryloyl group in the molecule, and 
     (C) 0.01 to 5 parts by weight of an active energy ray-sensitive radical polymerization initiator with respect to 100 parts by weight of the total of components (A) and (B) or components (A), (B-1) and (B-2), 
     as well as a lens sheet comprising a lens section formed on at least one side of a transparent substrate using the composition.

TECHNICAL FIELD

The present invention relates to an active energy ray-curablecomposition and to a lens sheet, such as a prism sheet, used in liquidcrystal display devices and the like, a lenticular lens sheet or Fresnellens sheet used for projection television screens or a lenticular lenssheet used for stereoscopic photography and the like.

BACKGROUND ART

Lens sheets, which include prism sheets used as backlight units forliquid crystal display devices and the like, Fresnel sheets andlenticular lens sheets used as projection screens for projectiontelevisions and microfilm readers, and lenticular lens sheets used forstereoscopic photography and the like, have lens sections which areformed using active energy ray-curing compositions. Such lens sheets areconstructed, for example, with a lens section made of a cured activeenergy ray-curable composition and formed on a transparent substrate.The active energy ray-curable composition used to form the lens sectionmust have a variety of properties, including adhesion to the transparentsubstrate, releasability from the lens mold, and suitable opticalcharacteristics.

For example, in the case of backlights used for the color liquid crystalpanels in color liquid crystal display devices for notebook computers,and in liquid crystal televisions and video-integrated liquid crystaltelevisions, it is important to reduce the power consumption to aminimum without lowering the brightness of the backlighting, andtherefore greater optical efficiency has been desired for backlights. Inthis regard there has been proposed a backlight with enhancedbrightness, wherein a prism sheet forming a prism row 2 on one side asshown in FIG. 1(a) is mounted on the light-emitting surface 4 of thebacklight 3 shown in FIG. 1(b) in the manner shown in FIG. 1(c).

The proposed prism sheet is usually one with a prism row formed by pressworking on one side of a transparent sheet made of a thermoplasticresin, or one with a prism row, made of an ultraviolet ray-curablecomposition, formed on one side of a transparent sheet, with thematerial for the former being polymethyl methacrylate (refractive index:1.49) or polycarbonate (refractive index: 1.59) and the ultravioletcuring composition used for the latter being a (meth)acrylate-basedcomposition (refractive index of cured resin: 1.49-1.55).

However, with methods of fabricating prism sheets by the pressureformation method using thermoplastic resin transparent sheets, it isdifficult to achieve a balance between the refractive index andtransparency of the materials to make a prism sheet with a satisfactorybrightness-enhancing effect, and with methods of fabricating prismsheets using the latter ultraviolet curing (meth)acrylate-basedcompositions, it becomes more difficult to sufficiently increase therefractive index the more the brightness of the prism sheet is enhanced.Furthermore, although prism sheets fabricated using materials with highrefractive indexes contribute greatly to increased front brightness ofthe backlight, a disadvantage has been that the light transmittancetends to be lowered.

DISCLOSURE OF THE INVENTION

As a result diligent research regarding active energy ray-curablecompositions in an attempt to overcome these problems of the prior art,the present inventors have found that it is possible to obtain curedproducts with high refractive indexes without causing lower lighttransmittance, to thus allow highly productive fabrication of lenssheets with a high brightness-enhancing effect, by using an acrylate ormethacrylate composition having a specific composition, and the presentinvention has thus been completed.

The present invention provides an active energy ray-curable compositioncomprising

(A) 20 to 80 parts by weight of a compound represented by the followinggeneral formula I: ##STR2## wherein R¹ represents hydrogen or methyl, Xand Y may be the same or different and represent methyl, chlorine,bromine or iodine, and t and u each independently represent an integerof 0-2,

(B) 20 to 80 parts by weight of at least one compound having at leastone acryloyl or methacryloyl group in the molecule, and

(C) 0.01 to 5 parts by weight of an active energy ray-sensitive radicalpolymerization initiator with respect to 100 parts by weight of thetotal of components (A) and (B).

The present invention further provides an active energy ray-curablecomposition comprising

(A) 20 to 80 parts by weight of a compound represented by generalformula I above,

(B-1) 10 to 90 parts by weight of at least one compound having at leasttwo acryloyl or methacryloyl groups in the molecule,

(B-2) 1 to 90 parts by weight of at least one monoacrylate ormonomethacrylate compound having one acryloyl or methacryloyl group inthe molecule, and

(C) 0.01 to 5 parts by weight of an active energy ray-sensitive radicalpolymerization initiator with respect to 100 parts by weight of thetotal of components (A), (B-1) and (B-2).

The present invention further provides a resin sheet comprising a lenssection formed using the above-mentioned active energy ray-curablecomposition on at least one side of a transparent substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a), (b) and (c) are schematic views illustrating a method offabricating a high-brightness backlight using a prism sheet and abacklight.

FIG. 2 is a partial cross-sectional view of a prism sheet.

FIG. 3 is a schematic view illustrating a method of forming a prismsheet.

FIG. 4 is a schematic cross-sectional view showing a prism sheet as itis taken out of a mold.

FIG. 5 is a perspective view of an embodiment of a mold for fabricationof a prism sheet.

A construction of a backlight 3, according to the present invention, isshown in FIGS. 1(a), (b) and (c). In the figure, a prism sheet 1,according to the present invention, has a row of multiple prisms 2formed on one surface using an active energy ray-curable composition.The backlight 3 comprises a plate-shaped photoconductor 7 and linearlight sources 5 such as a fluorescent light. The photoconductor 7 has alight-incident side surface and a light-emitting surface substantiallyperpendicular to the light-incident surfaces and a light-reflectivesurface is formed at the opposite side of the light-emitting surface.The linear light sources 5 are provided facing to the light-incidentside surfaces of the photoconductor 7 and are covered by alight-reflective sheets 6. The light from the linear light sources 5 isintroduced into the photoconductor 7 via the light-incident sidesurfaces and emitted from the light emitting surface 4 while a part ofthe incident light is reflected by the light-reflective surface.

BEST MODE FOR CARRYING OUT THE INVENTION

According to the present invention, the compound represented by generalformula I used as component (A) of the active energy ray-curablecomposition is a component which improves the refractive index withoutlowering the transmittance of the cured active energy ray-curablecomposition of the invention.

Typical specific examples of compounds which may constitute component(A) include bis(4-methacryloylthiophenyl)sulfide,bis(4-acryloylthiophenyl)sulfide,bis(4-methacryloylthio-3,5-dichlorophenyl)sulfide,bis(4-acryloylthio-3,5-dichlorophenyl)sulfide,bis(4-methacryloylthio-3,5-dibromophenyl)sulfide,bis(4-acryloylthio-3,5-dibromophenyl)sulfide,bis(4-methacryloylthio-3,5-dimethylphenyl)sulfide,bis(4-acryloylthio-3,5-dimethylphenyl)sulfide, etc., and these may beused alone or in combinations of 2 or more.

Of the above-mentioned compounds, bis(4-methacryloylthiophenyl)sulfideis most preferred.

Component (A) is used in a proportion in the range of 20-80 parts byweight, and preferably 30-70 parts by weight. If the content ofcomponent (A) is less than 20 parts by weight, then the low refractiveindex of the lens section of the lens sheet makes it difficult toproduce a lens sheet capable of exhibiting a high-brightness effect,whereas if the content of component (A) exceeds 80 parts by weight, notonly is the mechanical strength of the lens section lowered, butdeposition of the solid component (A) from the composition occurs duringstorage of the active energy ray-curing composition, which isundesirable as it tends to alter the composition.

Component (B), which is another compound having at least one(meth)acryloyl group in the molecule, is capable of dissolving the solidcomponent (A), and it is a component which serves to improve theproduction workability of the lens section during fabrication of thelens sheet from the composition of the invention. Formation of a lenssheet using an active energy ray-curable composition according to theinvention must allow excellent transfer of the lens shape of the lensmold, and component (B) is preferably a liquid at room temperature, witha low viscosity. Component (B) also preferably has a high dissolvingpower with respect to component (A). The type and amount of component(B) is selected based on the content of component (A), and the type anduse of component (B) is also determined with consideration to thecastability of the active energy ray-curable composition into the lensmold and its adhesion with the substrate sheet, as well as therefractive index of the lens section of the resulting lens sheet.

In cases where component (A) is used in a high proportion in thecomposition of the present invention, sedimentation of component (A)during storage may be minimized by using a compound having an aromaticgroup in the molecular structure as the molecule of component (B). Also,in order to improve the aforementioned castability of the composition ofthe invention into the lens mold, a low viscosity ester monomer ispreferably used in combination therewith to adjust the viscosity of theactive energy ray-curable composition.

Component (B) may be a mono- or poly(meth)acrylate of an aliphatic,alicyclic or aromatic mono- or polyalcohol, an aliphatic, alicyclic oraromatic urethane poly(meth)acrylate, epoxy poly(meth)acrylate,polyester poly(meth)acrylate, etc.

Specific examples of these include monofunctional ester (meth)acrylatessuch as methyl (meth)acrylate, butyl (meth)acrylate, 2-hydroxyethyl(meth)acrylate, cyclohexyl (meth)acrylate, tetrahydrofurfuryl(meth)acrylate, dicyclopentanyl (meth)acrylate, dicyclopentenyl(meth)acrylate, benzyl (meth)acrylate, phenyl (meth)acrylate,phenoxyethyl (meth)acrylate, 3-phenyl-2-hydroxypropyl (meth)acrylate,ortho-biphenyl (meth)acrylate, 3-(2,4-dibromophenyl)-2-hydroxypropyl(meth)acrylate, 2,4,6-tribromophenoxyethyl (meth)acrylate, etc.;bifunctional ester (meth)acrylates such as 1,6-hexanedioldi(meth)acrylate, neopentylglycol di(meth)acrylate, ethyleneglycoldi(meth)acrylate, polyethyleneglycol (n=2-15) di(meth)acrylate,polypropyleneglycol (n=2-15) di(meth)acrylate, polybutyleneglycol(n=2-15) di(meth)acrylate, 2,2-bis(4-(meth)acryloxyethoxyphenyl)propane, 2,2-bis(4-(meth)acryloxydiethoxyphenyl) propane,2,2-bis(4-(meth)acryloxyethoxy-3,5-dibromophenyl) propane,bis(4-(meth)acryloxydiethoxyphenyl) sulfone, etc.; polyfunctional ester(meth)acrylates such as trimethylolpropane tri(meth)acrylate,pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate,dipentaerythritol penta(meth)acrylate, dipentaerythritolhexa(meth)acrylate, etc.; epoxy di(meth)acrylates obtained by reactingbisphenol A-type diglycidyl ethers and (meth)acrylic acid; epoxydi(meth)acrylates obtained by reacting tetrabromobisphenol A-typediglycidyl ethers and (meth)acrylic acid; urethane di(meth)acrylateobtained by reacting tolylene diisocyanate and 2-hydroxypropyl(meth)acrylate; urethane di(meth)acrylates obtained by reacting xylylenediisocyanates and 2-hydroxyethyl (meth)acrylate; and urethanedi(meth)acrylates obtained by reacting isophorone diisocyanates and2-hydroxypropyl(meth)acrylate.

These monomers may be used alone or in mixtures of 2 or more.

Component (B) is used in a proportion of 20 to 80 parts by weight, andpreferably 30 to 70 parts by weight. A content of component (B) of lessthan 20 parts by weight is not preferred because not only is themechanical strength of the lens section lowered, but deposition of thesolid component (A) from the composition occurs during storage of theliquid composition, whereas a content of component (B) exceeding 80parts by weight is also not preferred because the low refractive indexof the lens section of the lens sheet makes it difficult to produce alens sheet capable of exhibiting a high-brightness effect.

The active energy ray-curable composition of the present inventionemploys both of the aforementioned components (A) and (B) to obtain acured product with no reduction in transparency and with a highrefractive index of 1.56 or greater, and is thus suitable for use as amaterial for constructing lens sections of prism sheets, Fresnel lenssheets and lenticular lens sheets.

Also, the active energy ray-sensitive radical polymerization initiatoras component (C) is preferably one which generates radicals whenirradiated by active energy rays, typical of which are ultraviolet raysand visible light rays, and any publicly known one may be used, with noparticular restrictions.

Specific examples of component (C) are radical polymerization initiatorswhich are sensitive to visible light rays, including carbonyl compoundssuch as benzoin, benzoin monomethyl ether, benzoin isopropyl ether,acetoin, benzyl, benzophenone, p-methoxybenzophenone,diethoxyacetophenone, 2,2-dimethoxy-1,2,-diphenylethan-1-one,2,2-diethoxyacetophenone, 1-hydroxycyclohexyl phenyl ketone,methylphenyl glyoxylate, ethylphenyl glyoxylate,2-hydroxy-2-methyl-1-phenylpropan-1-one and2-methyl-1-(4-(methylthio)phenyl)-2-morpholinopropanone-1; sulfurcompounds such as tetramethylthiuram monosulfide and tetramethylthiuramdisulfide; acylphosphine oxides such as2,4,6-trimethylbenzoyldiphenylphosphine oxide; camphor quinone,bis(cyclopentadienyl)-bis(2,6-difluoro-3-(pyr-1-yl) titanium, etc.

These compounds for component (C) may be used alone or in mixtures of 2or more.

Preferred among these compounds for component (C) are methylphenylglyoxylate, 2-hydroxy-2-methyl-1-phenylpropan-1-one,1-hydroxycyclohexylphenyl ketone, 2,2-dimethoxy-1,2-diphenylethan-1-one,benzyldimethyl ketal and 2,4,6-trimethylbenzoyldiphenylphosphine oxide.

Component (C) is used in a proportion in the range of 0.01 to 5 parts byweight, and preferably 0.02 to 3 parts by weight, with respect to 100parts by weight of the total of components (A) and (B). Curablecompositions with less than 0.01 part by weight of component (C) haveinsufficient curing properties, and component (C) is preferably presentin the composition at no more than 5 parts by weight, because thiscauses yellowing of the lens section of the lens sheet formed by thecomposition.

As mentioned above, the composition of the invention may contain,instead of the aforementioned component (B), 10 to 90 parts by weight ofcomponent (B-1) consisting of at least one compound having at least twoacryloyl or methacryloyl groups in the molecule, and 1 to 90 parts byweight of component (B-2) consisting of at least one monoacrylate ormonomethacrylate compound having one acryloyl or methacryloyl group inthe molecule.

Useful compounds as component (B-1) include compounds represented by thefollowing general formula II: ##STR3## wherein R² represents hydrogen ormethyl, V and W are the same or different and represent methyl,chlorine, bromine or iodine, p and q each independently represent aninteger of 0-2, R³ represents ##STR4## m and n represent integers of0-5, and Z represents ##STR5##

Component (B-1) is a component which improves the mechanical strength ofthe lens section of the lens sheet produced with the composition of theinvention. When forming the active energy ray-curable composition of theinvention into a lens sheet, it is cast between a substrate sheet and alens mold which transfers the lens shape, and it must be drawn to aprecise thickness and must allow proper transfer of the lens mold shape,while component (B-1) preferably is a liquid at room temperature and hasa low viscosity. Also, the solid component (A) preferably has a highdegree of solubility in the component.

Specific examples of compounds constituting component (B-1) includeester monomers such as 2,2-bis(4-(meth)acryloyloxyphenyl)-propane,2,2-bis(4-(meth)acryloyloxyethoxyphenyl)-propane,2,2-bis(4-(meth)acryloyloxydiethoxyphenyl)-propane,2,2-bis(4-(meth)acryloyloxytriethoxyphenyl)-propane,2,2-bis(4-(meth)acryloyloxytetraethoxyphenyl)-propane,2,2-bis(4-(meth)acryloyloxypentaethoxyphenyl)-propane,2,2-bis(4-(meth)acryloyloxyethoxy-3,5-dibromophenyl)-propane,2,2-bis(4-(meth)acryloyloxydiethoxy-3,5-dibromophenyl)-propane,bis(4-(meth)acryloyloxypentaethoxy-3,5-dibromophenyl)-propane,bis(4-(meth)acryloyloxyphenyl)-methane,bis(4-(meth)acryloyloxyethoxyphenyl)-methane,bis(4-(meth)acryloyloxydiethoxyphenyl)-methane,bis(4-(meth)acryloyloxytriethoxyphenyl)-methane,bis(4-(meth)acryloyloxytetraethoxyphenyl)-methane,bis(4-(meth)acryloyloxypentaethoxyphenyl)-methane,bis(4-(meth)acryloyloxydiethoxyphenyl)-sulfone,bis(4-(meth)acryloyloxypentaethoxyphenyl)-sulfone,bis(4-(meth)acryloyloxydiethoxyphenyl)-sulfide,bis(4-(meth)acryloyloxypentaethoxyphenyl)-sulfide,bis(4-(meth)acryloyloxydiethoxy-3,5-dimethylphenyl)-sulfide andbis(4-(meth)acryloyloxypentaethoxy-3,5-dimethylphenyl)-sulfide; andreaction products of bisphenol A-type epoxy compounds and methacrylicacid, reaction products of brominated bisphenol A-type epoxy compoundsand methacrylic acid, reaction products of bisphenol F-type epoxycompounds and methacrylic acid and reaction products of bisphenol S-typeepoxy compounds and methacrylic acid.

Any one of these monomers may be used alone, or 2 or more of them may beused in admixture.

Preferred among these compounds for component (B-1) are2,2-bis(4-methacryloyloxyethoxyphenyl)-propane,2,2-bis(4-acryloyloxyethoxyphenyl)-propane,2,2-bis(4-methacryloyloxydiethoxyphenyl)-propane,2,2-bis(4-acryloyloxydiethoxyphenyl)-propane,2,2-bis(4-methacryloyloxytriethoxyphenyl)-propane,2,2-bis(4-acryloyloxytriethoxyphenyl)-propane,2,2-bis(4-methacryloyloxytetraethoxyphenyl)-propane,2,2-bis(4-acryloyloxytetraethoxyphenyl)-propane,2,2-bis(4-methacryloyloxypentaethoxyphenyl)-propane and2,2-bis(4-acryloyloxypentaethoxyphenyl)-propane.

Component (B-1) is used in a proportion in the range of 10 to 90 partsby weight, and preferably 30 to 70 parts by weight. A content ofcomponent (B-1) of less than 10 parts by weight is not preferred becausethe transparency and mechanical strength of the lens section of the lenssheet is lowered, while deposition of the solid component (A) occursduring storage of the liquid composition tending to cause changes in thecomposition, whereas compositions with a content of component (B-1)exceeding 90 parts by weight do not give lens sections with sufficientlyhigh refractive indexes.

Compounds useful as component (B-2) include compounds represented by thefollowing general formulas III, IV, V and VI: ##STR6## wherein R⁴represents hydrogen or methyl, G represents methyl, chlorine, bromine oriodine, i represents an integer of 0-5, j represents an integer of 0-4,R⁵ represents ##STR7## and m and n represent integers of 0-5.

Component (B-2) is a component which serves to adjust the viscosity ofthe active energy ray-curable composition of the invention and therefractive index of the cured composition. When forming the activeenergy ray-curable composition of the invention into a lens sheet,castability into the lens mold, control of the thickness of the lenssheet and excellent transferability of the lens shape are crucial, andthe use of component (B-2) improves the thickness control andtransferability of the active energy ray-curable composition.Consequently, component (B-2) preferably is a liquid at room temperatureand has a low viscosity.

Specific examples of compounds composing component (B-2) include phenyl(meth)acrylate, benzyl (meth)acrylate, phenoxyethyl (meth)acrylate,phenoxy-2-methylethyl methacrylate, phenoxyethoxyethyl (meth)acrylate,3-phenoxy-2-hydroxypropyl (meth)acrylate, 2-phenylphenyl (meth)acrylate,4-phenylphenyl (meth)acrylate, 2-phenylphenoxyethyl (meth)acrylate,3-(2-phenylphenyl)-2-hydroxypropyl (meth)acrylate, 2-bromophenoxyethyl(meth)acrylate, 2,4-dibromophenoxyethyl (meth)acrylate,2,4,6-tribromophenoxyethyl (meth)acrylate and 2-bromobenzyl(meth)acrylate.

Any one of these monomers may be used alone, or 2 or more of them may beused in admixture.

Preferred among these are phenyl (meth)acrylate, benzyl (meth)acrylate,phenoxyethyl (meth)acrylate, phenoxy-2-methylethyl (meth)acrylate,phenoxyethoxyethyl (meth)acrylate and 3-phenoxy-2-hydroxypropyl(meth)acrylate.

Component (B-2) is used in a proportion in the range of 1 to 50 parts byweight, and preferably 5 to 30 parts by weight. With a content ofcomponent (B-2) of less than 1 part by weight, sufficient viscosityadjustment of the active energy ray-curable composition cannot beaccomplished, the castability into the lens mold is impaired, adhesionbetween the substrate sheet and lens section is lowered, and thicknesscontrol of the lens sheet and transferability of the lens shape arehindered. Lens sheets made from compositions with a content of component(B-2) exceeding 50 parts by weight are not preferred because they notonly have lower refractive indexes, but this amount also results inlower mechanical strength of the lens section.

Here as well, component (C) is used in a proportion in the range of 0.01to 5 parts by weight, and preferably 0.02 to 3 parts by weight, withrespect to 100 parts by weight of the total of components (A), (B-1) and(B-2).

For improved solubility of component (A) and improved adhesion of thelens section formed by the composition onto the substrate sheet, acompound with a radical polymerizing functional group, other than a(meth)acrylate, may also be added to the active energy ray-curablecomposition of the present invention in a range which does not inhibitthe effect of the invention. Specific examples of such compounds includestyrene, styrene derivatives such as divinylbenzene, chlorostyrene,dibromostyrene, etc., allyl compounds such as diallyl phthalate,diethyleneglycol bis(allylcarbonate), etc. and fumaric acid derivativessuch as dibenzyl fumarate, dibutyl fumarate, etc.

The active energy ray-curable composition of the invention may alsocontain, if necessary, other additives including anti-oxidants,anti-yellowing agents, ultraviolet absorbers, blueing agents, pigments,suspending agents, anti-foaming agents, anti-static agents, anti-foggingagents, and the like.

A lens sheet according to the invention employing an active energyray-curable composition will now be explained with reference to thedrawings.

The lens sheet of the invention comprises a transparent substrate sheet8 and a lens section 9 such as a prism row made of a polymer obtained bycuring an active energy ray-curable composition according to theinvention, as shown in FIG. 2. The material of the transparent substratesheet 8 may be a flexible glass plate which transmits ultraviolet rays,but for greater manageability there is preferably used a transparentsynthetic resin sheet with a thickness of a few hundred μm, usually madeof an acrylic resin, polycarbonate resin, vinyl chloride resin,polymethacrylimide resin or polyester resin. Particularly preferred aresheets made of polymethyl methacrylate, mixtures of polymethylmethacrylate and polyvinylidene fluoride-based resins, polycarbonateresins, polyethylene terephthalate and other polyester resins, whichhave relatively low refractive indexes and low surface reflectivity, andspecifically, those with refractive indexes lower than the refractiveindex of the lens section are preferred. In such cases, an anchorcoat-treated layer 10 may be provided for improved adhesion between thetransparent substrate sheet 8 and the lens section 9 made by curing theactive energy ray-curable composition, to obtain a lens sheet withstronger adhesion between the lens section 9 and the transparent sheet8.

For production of the lens sheet of the invention, as shown in FIG. 3,an active energy ray-curable composition 12 is cast and spread in a lensmold 11 which is made of metal, glass or resin and forms the lens shapeof the prism row, etc., after which a transparent substrate sheet 8 isplaced onto it and active energy rays from an active energy ray-emittingsource are irradiated through the sheet 8 for curing. As shown in FIG.4, the prepared lens sheet is then released from the lens mold 11 toobtain the lens sheet of the invention.

The active energy ray-emitting source may be a chemical reaction lamp,low pressure mercury lamp, high pressure mercury lamp, metal halidelamp, visible light halogen lamp, the suns rays, etc. The irradiation isconducted to a cumulative radiation energy of 0.1 to 50 J/cm² at awavelength of 200-600 nm. The environment for irradiation of the activeenergy rays may be air, nitrogen or an inert gas such as argon.

The lens mold 11 used is one such as shown in FIG. 5, which, in the caseof a prism sheet for example, is a prism mold 11 prepared with a prismshape having the vertical angle α shown in the insert circle. The lensmold may be made of a metal material such as aluminum, brass or steel,or a synthetic resin such as a silicone resin, urethane resin, epoxyresin, ABS resin, fluororesin or polymethylpentene resin, and/or themold may be produced by plating any of the above-mentioned materials ormixing them with various material powders, but metal molds are preferredfor use from the standpoint of heat resistance and strength.

The lens sheet obtained in the manner described above preferably has alens section made of the cured active energy ray-curable compositionwith a high refractive index of 1.60 or higher, and more preferably 1.62or higher. This is because a refractive index of the lens section, i.e.of the cured active energy ray-curable composition, of less than 1.60will tend to result in insufficient enhancement in front brightness whenit is used in a prism sheet for a backlight unit, for example. Also,when the lens sheet of the invention is used as a prism sheet for abacklight unit, a vertical angle α of the prism row in the range of 80°to 150° is preferred from the standpoint of the brightness-improvingeffect, and a range of 85° to 130° is more preferred.

The present invention is explained in more detail below by way of thefollowing examples and comparative examples. The abbreviations of themonomers given in the examples and tables are as follows.

MPSMA: bis(methacryloylthiophenyl)sulfide (trade name: MPSMA, SumitomoSeika)

BP-1: 2,2-bis(methacryloxyethoxyphenyl)propane (trade name: NK esterBPE-100N, Shin-Nakamura Chemical Co.)

POA: phenoxyethylacrylate (trade name: Viscoat #192, Osaka OrganicChemical Industries)

HMPO: 2-hydroxy-2-methyl-1-phenylpropan-1-one (trade name: Darocure1173, Ciba-Geigy)

PEDA: polyethyleneglycol diacrylate (trade name: NK ester A-400,Shin-Nakamura Chemical Co.)

BPA-2: 2,2-bis(methacryloxyethoxyphenyl)propane (trade name: NK esterA-BPE-4, Shin-Nakamura Chemical Co.)

BPM-5: 2,2-bis(methacryloxypentaethoxyphenyl)propane (trade name:Fancryl FA-321M, Hitachi Kasei)

BZM: benzyl methacrylate (trade name: Acriester BZ, Mitsubishi Rayon)

PHM: phenyl methacrylate (Mitsubishi Rayon)

PSM-1: phenylthioethyl methacrylate (synthetic product)

PSM-2: phenylthioethoxyethyl methacrylate (synthetic product)

HMPO: 2-hydroxy-2-methyl-1-phenylpropan-1-one (trade name: Darocure1173, Ciba Geigy)

APO: 2,4,6-trimethylbenzoyldiphenylphosphine oxide (trade name: LucilinTPO, BASF Co.)

PET: polyester film (Toyobo Co., A4100, thickness: 188 μm)

PMMA: polymethyl methacrylate resin board (Mitsubishi Rayon, Acrylate L,thickness: 0,8 mm)

PC: polycarbonate resin board (Mitsubishi Gas Chemical Co., thickness:0.5 mm)

EXAMPLE 1

(Preparation of Active Energy Ray-curable Composition)

A mixture of 50 g MPSMA, 30 g BP-1, 20 g POA and 2 g HMPO was heated to50° C. and stirred to obtain a transparent uniform solution.

(Fabrication of Prism Sheet)

The mixed solution was cast into an approximately A4-sized brass prismmold 11 having a triangular pyramid prism row with a prism row pitch of50 μm and a vertical angle α of 95° as shown in FIG. 5, and then, asshown in FIG. 3, a polyester film 8 of approximately the same size wasplaced over the cast side of the mixed solution, after which a 6.4 KW(80 W/cm) high-pressure mercury lamp situated 300 mm above the polyesterfilm was used to irradiate 1.2 J/cm² of ultraviolet ray energy for 6seconds to cure the mixed solution-applied layer, and this was releasedfrom the mold as shown in FIG. 4 to obtain a prism sheet.

(Evaluation of Active Energy Ray-curable Composition and Prism Sheet)

The prism sheet obtained in the above manner was evaluated according tothe following methods, and the results are given in Table 1.

(1) Transparency of composition

The transparency of the composition prepared in the manner describedabove was judged visually as follows:

∘: transparent

x: turbid, cloudy

(2) Castability of Composition into Mold

The castability of the composition into the mold and laminability of thesubstrate sheet were judged as follows:

∘: easy to cast

x: inclusion of bubbles, difficult to cast

(3) Measurement of Prism Refractive Index

For measurement of the refractive index of the prism section of theprism sheet obtained in the manner described above, the composition wascast into a mold prepared by inserting a gasket between two glass disks65 mm in diameter and 3 mm in thickness to form a 1 mm gap and windingthe perimeter fast with adhesive tape, and after one side of the glassmold was cured by irradiation with 10 J/cm² of ultraviolet ray energyfor 50 seconds using a high-pressure mercury lamp in the same manner asbefore, the cured resin board was released from the glass mold and itsrefractive index was measured with an Abbe refractometer using a sodiumD-ray light source at 20° C.

(4) Measurement of Brightness Enhancement

A prism sheet 1 having the construction shown in FIG. 1(a) fabricated inthe manner described above was set on the backlight unit 3 shown in FIG.1(b) comprising a Stanley cold-cathode tube 5, a reflective film 6, aMitsubishi Rayon acrylic photoconductor 7 and a Toray dispersion film 4,and the brightness was measured at a position 1 m directly above thebacklight unit, using a Topcon Model BM7 brightness photometer. Table 1gives the relative brightnesses with respect to 1.00 as the brightnesswith no prism sheet on the backlight surface.

(5) Adhesion

A razor was used to make 11 cuts both lengthwise and widthwise spaced1.5 mm apart on the prism row side of the prism sheet, each reaching thesubstrate film, to make 100 sections, after which cellophane adhesivetape (25 mm wide, product of Nichiban) was adhered onto the prism sideand then rapidly peeled off, and the number of remaining sections on theprism row side were counted.

EXAMPLES 2-7 Comparative Examples 1-2

Prism sheets were prepared in the same manner as Example 1, except forusing the compositions and substrate sheets shown in Table 1, and theresults, evaluated in the same manner as Example 1, are also given inTable 1.

Comparative Example 3

After placing a 0.8 mm-thick PMMA film over a stamping section with theprism shape of the prism mold 11 shown in FIG. 5, the film having aslightly smaller size than the area of the stamping section, a 3mm-thick polished stainless steel plate was placed on the laminatedproduct and, together with the mold, was subjected to a uniform 50 tload while heating to 180° C., and after 3 hours of standing forcooling, it was released to prepare a PMMA prism sheet. The brightnessof the prism sheet was then measured in the same manner as Example 1.The refractive index of the PMMA film was n=1.492.

Table 1 gives the relative brightnesses for Examples 1-7 and ComparativeExamples 1-3 with respect to 1.00 as the brightness of a backlight withno prism sheet. The brightnesses of the backlights employing prismsheets obtained in the examples of the invention all clearly demonstratemore excellent brightness-enhancing effects than the comparativeexamples.

The absolute values for the relative brightnesses shown in Table 1 varydepending on the different backlights used, but in this test conductedby the present inventors, there was no reversal of the size relationshipbetween the above-mentioned comparative examples and examples uponchanging the type of backlight.

                                      TABLE 1                                     __________________________________________________________________________                          Trans-                                                                            Casta-                                                                    parency                                                                           bility                                              Mixing amounts of each                                                                          Sub-                                                                              of  of  Refrac-                                         component (g)     strate                                                                            compo-                                                                            compo-                                                                            tive                                                                              Bright-                                                                           Adhe-                                   (A)      (B)  (C) sheet                                                                             sition                                                                            sition                                                                            index                                                                             ness                                                                              sion                                    __________________________________________________________________________    Ex. 1                                                                             MPSMA                                                                              BP-1(30)                                                                           HMPO                                                                              PET ∘                                                                     ∘                                                                     1.62                                                                              1.52                                                                              100                                         (50) POA(29)                                                                            (2)                                                             Ex. 2                                                                             MPSMA                                                                              BP-1(10)                                                                           HMPO                                                                              PET ∘                                                                     ∘                                                                     1.65                                                                              1.61                                                                              100                                         (70) POA(20)                                                                            (2)                                                             Ex. 3                                                                             MPSAM                                                                              BP-1(40)                                                                           HMPO                                                                              PET ∘                                                                     ∘                                                                     1.60                                                                              1.49                                                                              100                                         (30) POA(30)                                                                            (2)                                                             Ex. 4                                                                             MPSMA                                                                              PEDA(20)                                                                           HMPO                                                                              PET ∘                                                                     ∘                                                                     1.60                                                                              1.48                                                                              100                                         (40) POA(40)                                                                            (2)                                                             Ex. 5                                                                             MPSMA                                                                              BP-1(30)                                                                           HMPO                                                                              PMMA                                                                              ∘                                                                     ∘                                                                     1.62                                                                              1.53                                                                              100                                         (50) POA(20)                                                                            (2)                                                             Ex. 6                                                                             MPSMA                                                                              BP-1(30)                                                                           HMPO                                                                              PC  ∘                                                                     ∘                                                                     1.62                                                                              1.51                                                                              100                                         (50) POA(20)                                                                            (2)                                                             Ex. 7                                                                             MPSMA                                                                              BP-1(50)                                                                           HMPO                                                                              PET ∘                                                                     ∘                                                                     1.57                                                                              1.45                                                                              100                                         (15) POA(35)                                                                            (2)                                                             Comp.                                                                             --   BP-1(60)                                                                           HMPO                                                                              PET ∘                                                                     ∘                                                                     1.55                                                                              1.43                                                                              100                                     Ex. 1    POA(40)                                                                            (2)                                                             Comp.                                                                             MPSMA                                                                              POA(5)                                                                             HMPO                                                                              PET x   x   not not 0                                       Ex. 2                                                                             (95)      (2)             meas-                                                                             meas-                                                                     urable                                                                            urable                                      Comp.                                                                             --   --   --  PMMA                                                                              --  --  1.49                                                                              1.39                                                                              0                                       Ex. 3                                                                         __________________________________________________________________________

EXAMPLE 8

(Preparation of Active Energy Ray-curable Composition)

A mixture of 40 g MPSMA, 50 g BPA-2, 10 g BZM and 2 g HMPO was heated to50° C. and stirred to obtain a transparent uniform solution.

(Fabrication of Prism Sheet)

The mixed solution was cast into an approximately A4-sized brass prismmold 11 having a triangular pyramid prism row with a prism row pitch of50 μm and a vertical angle α of 95° as shown in FIG. 5, and then, asshown in FIG. 3, a PET film 8 of approximately the same size was placedover the cast side of the mixed solution, after which a 6.4 KW (80 W/cm)high-pressure mercury lamp situated 300 mm over the PET film was used toirradiate 1.2 J/cm² of ultraviolet ray energy for 6 seconds to cure themixed solution-applied layer, and this was released from the mold asshown in FIG. 4 to obtain a prism sheet.

(Evaluation of Active Energy Ray-curable Composition and Prism Sheet)

The prism sheet obtained in the above manner was evaluated, according tothe methods in Example 1, and the results are given in Table 2.

EXAMPLES 9-13 Comparative Examples 4-6

Prism sheets were prepared in the same manner as Example 8, except forusing the compositions and substrate sheets listed in Table 2, and theresults evaluated in the same manner as Example 8 are also given inTable 2. For reference, the results of Comparative Example 3 are alsogive in Table 2.

Table 2 gives the relative brightnesses for Examples 8-13 andComparative Examples 3-6 with respect to 1.00 as the brightness of abacklight with no prism sheet. The brightnesses of the backlightsemploying prism sheets obtained in the examples of the invention allclearly demonstrate more excellent brightness-enhancing effects than thecomparative examples.

The absolute values for the relative brightnesses shown in Table 2 varydepending on the different backlights used, but in this test conductedby the present inventors, there was no reversal of the size relationshipbetween the above-mentioned comparative examples and examples bychanging the type of backlight.

                                      TABLE 2                                     __________________________________________________________________________                              Trans-                                                                            Casta-                                                                    parency                                                                           bility                                          Mixing amounts of each                                                                              Sub-                                                                              of  of  Refrac-                                     component (g)         strate                                                                            compo-                                                                            compo-                                                                            tive                                                                              Bright-                                                                           Adhe-                               (A)      (B-1)                                                                              (B-2)                                                                             (C) sheet                                                                             sition                                                                            sition                                                                            index                                                                             ness                                                                              sion                                __________________________________________________________________________    Ex. 8                                                                             MPSMA                                                                              BPA-2                                                                              BZM HMPO                                                                              PET ∘                                                                     ∘                                                                     1.62                                                                              1.52                                                                              100                                     (40) (50) (10)                                                                              (2)                                                         Ex. 9                                                                             MPSMA                                                                              BPA-2                                                                              PZM HMPO                                                                              PET ∘                                                                     ∘                                                                     1.66                                                                              1.63                                                                              100                                     (70) (15) (15)                                                                              (2)                                                         Ex. 10                                                                            MPSAM                                                                              BPA-2                                                                              BZM HMPO                                                                              PET ∘                                                                     ∘                                                                     1.60                                                                              1.49                                                                              100                                     (30) (60) (10)                                                                              (2)                                                         Ex. 11                                                                            MPSMA                                                                              BPA-5                                                                              PHM APO PET ∘                                                                     ∘                                                                     1.63                                                                              1.62                                                                              100                                     (45) (20) (35)                                                                              (2)                                                         Ex. 12                                                                            MPSMA                                                                              BPA-2                                                                              BZM HMPO                                                                              PMMA                                                                              ∘                                                                     ∘                                                                     1.62                                                                              1.53                                                                              100                                     (40) (50) (10)                                                                              (2)                                                         Ex. 13                                                                            MPSMA                                                                              BPA-5                                                                              PHM APO PC  ∘                                                                     ∘                                                                     1.62                                                                              1.51                                                                              100                                     (45) (20) (35)                                                                              (2)                                                         Comp.                                                                             --   BPA-2                                                                              BZM HMPO                                                                              PET ∘                                                                     ∘                                                                     1.55                                                                              1.43                                                                              100                                 Ex. 4    (60) (40)                                                                              (2)                                                         Comp.                                                                             MPSMA                                                                              --   BZM HMPO                                                                              PET x   x   not not 0                                   Ex. 5                                                                             (95)      (5) (2)             meas-                                                                             meas-                                                                     urable                                                                            urable                                  Comp.                                                                             MPSMA                                                                              BPA-2                                                                              --  HMPO                                                                              PET ∘                                                                     x   1.63                                                                              1.60                                                                              50                                  Ex. 6                                                                             (50) (50)     (2)                                                         Comp.                                                                             --   --   --  --  PMMA                                                                              --  --  1.49                                                                              1.39                                                                              0                                   Ex. 3                                                                         __________________________________________________________________________

EXAMPLE 14

(Preparation of Active Energy Ray-curable Composition)

A mixture of 40 g MPSMA, 50 g BPA-2, 30 g PSM-1 and 2 g HMPO was heatedto 50° C. and stirred to obtain a transparent uniform solution.

(Fabrication of Prism Sheet)

The mixed solution was cast into an approximately A4-sized brass prismmold 11 having a triangular pyramid prism row with a pitch of 50 μm anda vertical angle α of 95° as shown in FIG. 5, and after application, anapproximately A4-sized PET film 8 was placed over it as shown in FIG. 3,after which a 6.4 KW (80 W/cm) high-pressure mercury lamp situated 300mm over the PET film was used to irradiate 1.2 J/cm² of ultraviolet rayenergy for 6 seconds for curing, and release from the mold gave a prismsheet provided with a prism row having a vertical angle of 90°.

(Evaluation of Active Energy Ray-curable Composition and Prism Sheet)

The active energy ray-curable composition and prism sheet produced inthe above manner was evaluated, according to the methods in Example 1,and the results are given in Table 3.

EXAMPLES 15-19 Comparative Examples 7-9

Prism sheets were prepared in the same manner as Example 14, except forusing the compositions and substrate sheets listed in Table 3, and theresults evaluated in the same manner as Example 14 are also given inTable 3. For reference, the results of Comparative Example 3 are alsogive in Table 3.

Table 3 gives the relative brightnesses for Examples 14-19 andComparative Examples 7-9 with respect to 1.00 as the brightness of abacklight with no prism sheet. The examples of the invention all clearlydemonstrate more excellent brightness-enhancing effects than thecomparative examples.

The absolute values for the relative brightnesses shown in Table 3 varydepending on the different backlights used, but in this study conductedby the present inventors, there was no reversal of the size relationshipbetween the above-mentioned comparative examples and examples bychanging the type of backlight.

                                      TABLE 3                                     __________________________________________________________________________                              Trans-                                                                            Casta-                                                                    parency                                                                           bility                                          Mixing amounts of each                                                                              Sub-                                                                              of  of  Refrac-                                     component (g)         strate                                                                            compo-                                                                            compo-                                                                            tive                                                                              Bright-                                                                           Adhe-                               (A)      (B-1)                                                                              (B-2)                                                                             (C) sheet                                                                             sition                                                                            sition                                                                            index                                                                             ness                                                                              sion                                __________________________________________________________________________    Ex. 14                                                                            MPSMA                                                                              BPA-2                                                                              PSM-1                                                                             HMPO                                                                              PET ∘                                                                     ∘                                                                     1.63                                                                              1.64                                                                              100                                     (40) (30) (30)                                                                              (2)                                                         Ex. 15                                                                            MPSMA                                                                              BPA-2                                                                              PSM-1                                                                             HMPO                                                                              PET ∘                                                                     ∘                                                                     1.66                                                                              1.65                                                                              100                                     (70) (15) (15)                                                                              (2)                                                         Ex. 16                                                                            MPSAM                                                                              BPA-2                                                                              PSM-2                                                                             HMPO                                                                              PET ∘                                                                     ∘                                                                     1.61                                                                              1.59                                                                              100                                     (30) (60) (10)                                                                              (2)                                                         Ex. 17                                                                            MPSMA                                                                              BPA-5                                                                              PSM-2                                                                             APO PET ∘                                                                     ∘                                                                     1.65                                                                              1.64                                                                              100                                     (50) (10) (40)                                                                              (2)                                                         Ex. 18                                                                            MPSMA                                                                              BPA-2                                                                              PSM-1                                                                             HMPO                                                                              PMMA                                                                              ∘                                                                     ∘                                                                     1.63                                                                              1.65                                                                              100                                     (40) (30) (30)                                                                              (2)                                                         Ex. 19                                                                            MPSMA                                                                              BPA-2                                                                              PSM-1                                                                             APO PC  ∘                                                                     ∘                                                                     1.63                                                                              1.63                                                                              100                                     (40) (30) (30)                                                                              (2)                                                         Comp.                                                                             --   BPA-2                                                                              PSM-1                                                                             HMPO                                                                              PET ∘                                                                     ∘                                                                     1.58                                                                              1.45                                                                              100                                 Ex. 7    (60) (40)                                                                              (2)                                                         Comp.                                                                             MPSMA                                                                              --   PSM-1                                                                             HMPO                                                                              PET x   x   not not 0                                   Ex. 8                                                                             (95)      (5) (2)             meas-                                                                             meas-                                                                     urable                                                                            urable                                  Comp.                                                                             MPSMA                                                                              BPA-2                                                                              PSM-1                                                                             HMPO                                                                              PET ∘                                                                     ∘                                                                     1.61                                                                              1.50                                                                              10                                  Ex. 9                                                                             (10) (10) (80)                                                                              (2)                                                         Comp.                                                                             --   --   --  --  PMMA                                                                              --  --  1.49                                                                              1.39                                                                              0                                   Ex. 3                                                                         __________________________________________________________________________

Industrial Applicability

As explained above, the active energy ray-curable composition of thepresent invention gives a cured product with a high refractive indexwithout reduction in transparency, and thus when used as a material forthe lens section of a lens sheet with the lens section formed on thesurface of a transparent substrate, it can notably enhance the frontbrightness of the lens sheet and provide a lens sheet with satisfactorymanageability and productivity.

We claim:
 1. A lens sheet, comprising a transparent substrate and a lenssection formed on at least one side of the transparent substrate whereinthe lens section is formed from an active energy ray-curable compositionand, wherein the active energy ray-curable composition comprises:(A) 20to 80 parts by weight of a compound represented by the following generalformula I: ##STR8## wherein R¹ represents hydrogen or methyl, X and Ymay be the same or different and represent methyl, chlorine, bromine oriodine, and t and u each independently represent an integer of 0-2,(B-1) 10 to 90 parts by weight of at least one compound represented bythe following general formula II: ##STR9## wherein R² representshydrogen or methyl, V and W may be the same or different and representmethyl, chlorine, bromine or iodine, p and q each independentlyrepresent an integer of 0-2, R³ represents ##STR10## m and n representintegers of 0-5, and z represents ##STR11## (B-2) 1 to 90 parts byweight of at least one monoacrylate or monomethacrylate compound havingone acryloyl or methacryloyl group in the molecule, and (C) 0.01 to 5parts by weight of an active energy ray-sensitive radical polymerizationinitiator with respect to 100 parts by weight of the total of saidcomponents (A), (B-1) and (B-2), andwherein said component (A) and (B-1)are different from each other.
 2. A lens sheet according to claim 1,wherein the compound constituting said component (A) isbis(4-methacryloylthiophenyl)sulfide.
 3. A lens sheet according to claim1, wherein the compound having at least two acryloyl or methacryloylgroups in the molecule, which constitutes said component (B-1), isselected from 2,2-bis(4-methacryloyloxyethoxyphenyl)-propane,2,2-bis(4-acryloyloxyethoxyphenyl)-propane,2,2-bis(4-methacryloyloxydiethoxyphenyl)-propane,2,2-bis(4-acryloyloxydiethoxyphenyl)-propane,2,2-bis(4-methacryloyloxytriethoxyphenyl)-propane,2,2-bis(4-acryloyloxytriethoxyphenyl)-propane,2,2-bis(4-methacryloyloxytetraethoxyphenyl)-propane,2,2-bis(4-acryloyloxytetraethoxyphenyl)-propane,2,2-bis(4-methacryloyloxypentaethoxyphenyl)-propane and2,2-bis(4-acryloyloxypentaethoxyphenyl)-propane.
 4. A lens sheetaccording to claim 1, wherein the monoacrylate or monomethacrylatecompound which constitutes said component (B-2) is selected fromcompounds represented by the following general formulas III, IV, V andVI: ##STR12## wherein R⁴ represents hydrogen or methyl, G representsmethyl, chlorine, bromine or iodine, i represents an integer of 0-5, jrepresents an integer of 0-4, R⁵ represents ##STR13## and m and nrepresent integers of 0-5.
 5. A lens sheet according to claim 4, whereinthe monoacrylate or monomethacrylate compound which constitutes saidcomponent (B-2) is selected from phenyl methacrylate, phenyl acrylate,benzyl methacrylate, benzyl acrylate, phenoxyethyl methacrylate,phenoxyethyl acrylate, phenoxy-2-methylethyl methacrylate,phenoxyethoxyethyl methacrylate, phenoxyethoxyethyl acrylate,3-phenoxy-2-hydroxypropyl methacrylate, 3-phenoxy-2-hydroxypropylacrylate, 2-phenylphenyl methacrylate, 2-phenylphenyl acrylate,4-phenylphenyl methacrylate, 4-phenylphenyl acrylate,2-phenylphenoxyethyl methacrylate, 2-phenylphenoxyethyl acrylate,3-(2-phenylphenyl)-2-hydroxypropyl methacrylate,3-(2-phenylphenyl)-2-hydroxypropyl acrylate, 2-bromophenoxyethylmethacrylate, 2-bromophenoxyethyl acrylate, 2,4-dibromophenoxyethylmethacrylate, 2,4-dibromophenoxyethyl acrylate, phenylthioethylmethacrylate, phenylthioethyl acrylate, phenylthioethoxy methacrylate,phenylthioethoxy acrylate, phenylthiodiethoxy methacrylate,phenylthiodiethoxy acrylate, 2-phenylphenylthioethyl methacrylate,2-phenylphenylthioethyl acrylate, 4-chlorophenylthioethyl methacrylate,4-chlorophenylthioethyl acrylate, benzylthioethyl methacrylate andbenzylthioethyl acrylate.
 6. The lens sheet according to claim 1,wherein the refractive index of said lens section is 1.60 or greater. 7.The lens sheet according to claim 1, wherein said lens section is in theform of a minute prism row.
 8. The lens sheet according to claim 7,wherein the vertical angle of each of the prisms of said minute prismrow is 80-150°.